PROJECT SUMMARY Elucidating the molecular basis of neurological and psychiatric disease is of critical importance for the design of improved therapies. One signaling pathway that has been identified as a hub of dysregulation in several brain disorders is the mTOR pathway. A large body of work has established that altered mTOR signaling affects key aspects of neuronal structure and function including cell morphology, connectivity, synaptic plasticity, and excitability. However, very little is known about the molecules downstream of mTOR responsible for regulating these processes. This information is essential as the effectors of mTOR signaling represent potential drug targets that could provide improved specificity compared with systemic mTOR blockade; a current treatment strategy that is associated with problematic side-effects. A central cellular process regulated by mTOR complex 1 (mTORC1) is protein synthesis. Therefore, identifying the translational targets of mTORC1 is a key first step towards understanding how this pathway controls neuronal function in both normal and disease states. Here we will generate a comprehensive and unbiased assessment of mTORC1's influence on protein synthesis in distinct disease-relevant cell types in the mouse brain. To do this we will genetically and pharmacologically manipulate mTORC1 signaling in vivo and perform translational profiling of defined classes of neurons. We will use a novel approach, called FLEX-TRAP, to express a tagged-ribosomal protein selectively in sub-populations of neurons and use translating ribosome affinity purification (TRAP) to isolate mRNAs that are being actively translated. These mRNAs will be analyzed using next generation sequencing and translational profiles will be compared across conditions and cell types. For this exploratory project we will examine the impact of mTORC1 signaling on the translational profile of four disease-relevant populations of neurons: hippocampal pyramidal cells, direct and indirect pathway striatal projection neurons, and midbrain dopamine neurons. This analysis will provide the first comprehensive and cell type-specific description of mTORC1's impact on mRNA translation in the brain and identify the molecules downstream of mTORC1 responsible for mediating changes in cellular and synaptic function.